(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to a LCD device including a homogeneously-oriented liquid crystal (LC) layer and having an improved viewing angle characteristic. The present invention also relates to a polarizing film pair for use in such a LCD device.
(b) Description of the Related Art
A lateral-electric-field-mode LCD device, such as an in-plane-switching-mode (IPS-mode) LCD device, is known in the art, wherein a pixel electrode and a common electrode generate therebetween a lateral electric field in the LC layer. The IPS-mode LCD device has the advantage of a wider viewing angle characteristic over the conventional TN-mode LCD device. The IPS-mode LCD device generally includes a LC cell, which includes first and second substrates and a LC layer sandwiched therebetween, and first and second polarizing films adhered onto the outer surface of the first and second substrates, respectively, of the LC cell.
The first and second substrates have a function of visualizing the change of orientation of LC molecules in the LC layer defined by the lateral electric field. The polarizing films are configured by layering a transparent protective film, such as a triacetyl cellulose (TAC) film, on a polarizing layer. The polarizing layer divides the incident light into two polarized light components perpendicular to one another, and passes therethrough one of the polarized-light components having an oscillation plane parallel to the transmission axis of the polarizing layer, while absorbing or scattering the other of the polarized-light components having an oscillation plane parallel to the absorption axis of the polarizing layer.
In the LCD device, the polarizing film pair and the initial orientation of the LC layer are generally designed such that the LCD device represents a dark state when both the electrodes generate therebetween no electric field whereas the LCD device represents a bright state when both the electrodes generate therebetween a specific electric field. In the bright state, the LC molecules in the LC layer are oriented at an angle of λ/2 deviated from the initial orientation of the LC molecules.
In general, a LCD device having a homogeneously-oriented LC layer, such as a transmission IPS-mode LCD device, includes a polarizing film pair sandwiching therebetween a LC cell as viewed in the thickness direction thereof. Both the polarizing films of the pair are disposed so that the transmission axes of both the polarizing films are perpendicular to one another. Such polarizing films are referred to as an orthogonal polarizer in the art. It is known that the orthogonal polarizer has an undesirable viewing angle dependency, wherein a light incident onto the orthogonal polarizer in a slanted direction with respect thereto changes the direction of the transmission axis of the polarizing film.
Thus, the orthogonal polarizer, which is disposed so that both the transmission axes of the polarizing layers are crossed at a right angle, allows the light incident onto the LC cell in a slanted direction and passed by the first polarizing film to have an undesirable light component parallel to the transmission axis of the second polarizing film due to deviation of the crossed angle from the right angle. The undesirable light component causes a leakage light passing through the second polarizing film upon display of a dark state in the LCD device.
The viewing angle dependency of the orthogonal polarizer as described above causes a reduction in the range of viewing angle, i.e., the visual range in which the luminance, contrast ratio and coloring of the image are well appreciated on the screen of the LCD device. In this view point, for achieving a wider range of viewing angle in the LCD device, it is essential to develop an optical-compensation polarizing film which is capable of reducing the viewing angle dependency of the orthogonal polarizer to prevent the leakage light and increase the range of viewing angle. Patent publication JP-2001-242462A, for example, describes a technique for use in a homogeneously-oriented IPS-mode LCD device, wherein a biaxial retardation film, acting as an optical compensating film, compensates the orthogonal polarizer so that the light component of the slanted-incident light parallel to the transmission axis of the second polarizing film is changed toward the perpendicular of the transmission axis, the slanted-incident light being incident onto the LC cell in a slanted viewing direction and passed by the first polarizing film.
In the LCD device described in the above publication, the mere addition of the optical compensating film having a wavelength dependency similarly to the LC layer cannot optically compensate the orthogonal polarizer in the entire wavelength range, although the compensation is effective in a specific wavelength range. This causes leakage light in the other wavelength ranges, and thus involves coloring of the image depending on the azimuth angle of the viewing direction. In addition, since the transparent protective layer of the polarizing film has a retardation depending on the thickness of the transparent protective film, the slanted-incident light, which is once turned by the light-incident-side polarizing layer to a linearly-polarized light, is turned again to an elliptically-polarized light by the transparent protective layer and additionally turned in the polarization by the LC layer, thereby increasing the undesirable leakage light and coloring of the emitted light.